This invention relates to television kinescope protection circuits and more particularly to blanking circuits.
Television kinescopes using an electron beam to illuminate a phosphor-coated screen generally have the electron beam scanned over the surface of the screen in horizontal and vertical directions to form an illuminated raster. If the electron beam is not scanned over the entire raster, but is repetitively scanned over only a single horizontal or vertical line, the phosphor may be damaged by the excessive localized power. U.S. Pat. No. 3,146,372 issued on Aug. 25, 1964 in the name of R. B. Fertig describes an arrangement by which the electron-beam deflection signal is rectified to change the bias of the grid to blank the screen when normal scanning raster ceases. This arrangement is relatively slow, because the rectified signal from beam deflecting apparatus must decay significantly before the beam is turned off or blanked.
A faster blanking arrangement is described in U.S. Pat. No. 3,308,333 issued on Mar. 7, 1967 to S. J. Lent. In the Lent arrangement, a monostable multivibrator unblanks the kinescope in its unstable or quasistable condition. The stable condition or state is initiated at the beginning of vertical retrace by application of a vertical synchronizing pulse to the multivibrator, and the unstable state is triggered by the lagging or trailing edge of the vertical flyback or retrace pulse. The unstable period of the multivibrator exceeds the vertical deflection interval. In the event that the vertical deflection fails while the multivibrator is in its stable state, the unstable state is not initiated and the multivibrator remains in its stable blanking condition. If deflection fails when the multivibrator is in its unstable condition, it will return to its stable state after the expiration of the natural period of the unstable condition.
However, it may not always be desired to initiate and terminate kinescope blanking precisely at the beginning and end of the vertical retrace interval. For example, it may be desired to transmit vertical-interval reference signals or digital information in the horizontal lines preceding or following the vertical retrace interval, and blanking of these lines may be desirable. When blanking is not to be initiated and terminated simultaneously with vertical retrace, the Lent protection arrangement may not be satisfactory.
An embodiment of a switched synchronous vertical deflection (SSVD) circuit is described in U.S. Pat. No. 4,048,544 issued on Sept. 13, 1977 in the name of Peter E. Haferl, in which the vertical deflection current is derived by integration of horizontal retrace pulses width-modulated by switches in a feedback manner under control of an analogue of the desired deflection current. If the horizontal retrace pulse drive to the SSVD circuit fails, the vertical deflection will fail though horizontal deflection continues. The SSVD circuit may have an inherent delay between application of the analogue of the deflection current and the resulting deflection current. Copending application Ser. No. 829,539 filed Aug. 31, 1977 in the name of Adel Ahmed describes a blanking pulse generator with delayed transition for use with the SSVD. It is desirable to blank the kinescope upon failure of the horizontal retrace pulse drive to the SSVD. It is further desirable to accomplish this in a manner compatible with the width-modulated control of the SSVD and with provision for kinescope blanking initiation and termination at times other than the leading edge of the vertical synchronizing pulse and lagging edge of the vertical retrace pulse, respectively.